There is a mathematical way I don't remember it but it has to do with the 1/4 wave of the longest distance from subs to mic location. It can get you close but every car is different
Or by using a meter you want to build a box with. A very even frequency response, meter it outside the car(big open space) then put it in the car and find the difference.
The peak won't always be the best frequency though sometimes a different frequency will gain better when more power is applied and will eventually pass up your vehicles resonant frequency.
This is we're you test test and test some more
rd d9 @ .5ohm
2 sa Zv1's 5.8cf tuned to 48 or 32 with port plugs
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